The present invention relates to electrical systems for motor vehicles and, more particularly, to a power delivery circuit for energetic starting of an engine in a vehicle having a pulsed charge starter/alternator system. Combined starter/alternators such as those known in the art are disclosed in U.S. Pat. Nos. 4,720,638; 4,916,345; 5,001,412; 5,097,140 and 5,469,820. A combined starter/alternator as used in a motor vehicle can be used as a motor to crank and start the engine as well as a generator to provide electrical power to the vehicle electrical systems. When operated as a starter motor, the starter/alternator is supplied with current from the battery and is operated to rotate the crankshaft of the engine. The engine crankshaft is rotated until the engine fires and continues to run on its own power. When used as a generator, the running engine is coupled to the starter/alternator which, in turn, produces a three-phase output that is rectified to a steady state DC output that is used to maintain the charge on the vehicle energy storage device to meet the vehicle electrical load requirements.
When operating as a generator, the efficiency of the starter/alternator is defined as the ratio of the input power to the output power. Such generators have both fixed and variable losses. Some of these losses are associated with the switching circuitry such as the inverter used to rectify the output of the generator. There are three primary classifications of losses: mechanical losses, electrical losses, and magnetic losses. Mechanical losses are primarily due to the rotation of the rotor and include bearing friction loss and windage loss. Magnetic losses include eddy current-current loss and hysteresis loss. All of these losses can be grouped into two categories, namely, fixed losses and variable losses. Fixed losses are losses that do not change with load when the machine is operating at a known speed. Therefore, rotational losses are one part of fixed losses. In the case of a wound field machine where the field current is required to set up the required flux in the machine, the power supplied to the field lining is also considered a fixed loss. On the other hand, variable losses are losses that vary with the load current. All of the copper or resistive losses are included in this category. Since the losses associated with the starter/alternator when operating as a generator are only partially load dependent, the system exhibits low efficiencies at low power levels. At higher output power levels, the output power rises relative to the losses and correspondingly, the system efficiency rises as well. This continues up to a peak efficiency torque point whereafter additional torque input into the generator does not result in a significant increase in power output and, hence, the efficiency falls off.
It has been found that operating a starter/alternator at a continuous power output approximately equal to the vehicle electrical load demand such as 500 W is a low efficiency output for a typical starter/alternator. Accordingly, there is a need for a starter/alternator control system having increased efficiency.
It is also desirable to provide a system that allows a higher amount of instantaneous power to the starter/alternator when operating as a starter motor.
It is therefore an object of the present invention to provide an improved power delivery circuit having increased starting power in a pulsed charge starter/alternator system.
According to the present invention, the foregoing and other objects are attained by a method of energizing a combined starter/alternator to start an engine in a vehicle including a primary energy storage device and a secondary energy storage device. The method includes the steps of monitoring a charge value on the secondary energy storage device, and activating a converter switching circuit to charge the secondary energy storage device with the primary energy storage device until the charge value is greater than a maximum charge value. Thereafter, an inverter switching circuit is activated to energize the starter/alternator with the power available in the primary and secondary energy storage devices. In another aspect of the invention, the starter/alternator is activated as a starter motor until the engine starts or the charge on the secondary energy storage device falls below a minimum charge value.
One advantage of the present invention is that it has improved efficiency as compared to conventional starter/alternator charging systems, and improved starting capacity as compared to conventional starter/alternator systems. Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.